1. Field of the Invention
The present invention relates to a pattern forming method and a pattern forming apparatus that form a conductive pattern on a base material by a liquid discharging method.
2. Description of the Related Art
Circuit boards on which semiconductors, such as LSIs, and various electronic components are mounted are now currently used as the cores of electronic apparatuses, communication apparatuses, and computers. In these circuit boards, a composite of a reinforcing material such as a ceramic or glass fiber, and a synthetic resin such as epoxy resin are frequently used as a base material. However, when a circuit board is incorporated in a small apparatus such as a mobile telephone or a digital camera, a base material is frequently made flexible by being formed of, for example, polyester resin or aramid resin in order to increase mountability. On the other hand, although one-sided boards and double-sided boards have been mainly used heretofore, multilayer circuit boards each including, for example, eight or sixteen circuit-pattern layers are currently dominating the market because size reduction can be achieved and the integration level of the apparatuses can be increased. In addition, the definition and density of circuit patterns have been rapidly increased with the increase in the speed of electronic circuits.
Various methods are used to form a circuit pattern on a circuit board. Japanese Patent Laid-Open No. 11-163499 discloses a method for forming a circuit pattern by a liquid discharging method (inkjet recording). The use of the liquid discharging method reduces the machining cost and overcomes an environmental problem of waste disposal, compared with known subtractive processes. Moreover, the liquid discharging method provides other advantages, for example, high-speed recording is possible, the running cost is low, space saving of the apparatus can be easily achieved, and noise is low. While the liquid discharging method is basically used for recording, it is promising for applications to various industrial fields, for example, formation of circuit patterns on circuit boards, formation of pixels in color filters, formation of luminescent layers of organic electroluminescent elements and electron emission elements of electron sources, and production of microlenses.
The above-described publication discloses that a pattern composed of a conductive pattern and an insulating pattern is formed as a circuit pattern on a base material by discharging a conductive-pattern solution and an insulating-pattern solution from a liquid discharging head.
However, the use of the liquid discharging method causes a new specific problem. That is, a circuit pattern is recorded by discharging a solution for the circuit pattern from discharge openings provided corresponding to recording elements (for example, heating elements or piezoelectric elements) in the liquid discharging method. If any of the recording elements does not perform discharging for some time, evaporation of the solution proceeds at the discharge opening corresponding to the recording element, and the residual component sometimes sticks to the adjacency of the discharge opening.
FIGS. 1A and 1B are schematic views showing states in which the above-described sticking is caused at a discharge opening. FIG. 1A shows a state in which one discharge opening is clogged with sticking solution, and FIG. 1B shows a state in which the solution sticks outside the discharge opening.
FIGS. 2A to 2C are schematic views showing adverse effects on a circuit pattern recorded with a recording head in which sticking of the solution occurs, as shown in FIGS. 1A and 1B. FIG. 2A shows a linear pattern normally recorded in a state in which the above-described clogging and sticking do not occur at any of a plurality of discharge openings of the recording head. FIG. 2B shows a linear pattern recorded in a state in which one of the discharge openings is clogged, as shown in FIG. 1A. Since discharging from the clogged discharge opening is impossible, no solution is applied in a portion of the linear pattern. This leads to a wire break of the linear pattern. Hereinafter, this state in which discharging of the solution is impossible is referred to as a nondischarging state.
FIG. 2C shows a linear pattern recorded in a state in which a foreign substance sticks to the rim of a discharge opening, as shown in FIG. 1B. Since discharging is performed in such a state in which the foreign substance sticks near the discharge opening, position error is caused by deviation of the discharging direction, and a solution droplet lands at a position displaced from a desired position on a base material. As a result, a wire break occurs in the circuit pattern. When the displaced landing droplet comes into contact with another pattern, a short circuit occurs. Hereinafter, landing of the solution droplet at an undesirable position will be referred to as dot misalignment. While the substance sticks near the discharge opening in FIGS. 1B and 2C, even when a part of discharged solution sticks near the discharge opening, dot misalignment is sometimes caused by the influence on the discharging direction of the solution from the discharge opening. When a wire break in a portion of the circuit pattern or a short circuit between adjacent circuit patterns is thus caused by nondischarging or dot misalignment in the recording head, it is fatal to the recorded circuit pattern.
A method for minimizing these adverse effects is disclosed in Japanese Patent Laid-Open No. 6-24008. In this method, a line pattern is recorded by using all discharge openings of a recording head, as shown in FIGS. 2A to 2C, and it is optically or electronically checked whether there is an unrecorded portion in the line pattern. When the unrecorded portion is detected, the recording head is subjected to recovery operation.
In recent circuit forming technologies, there is an increasing demand to further reduce the weight, thickness, and size of circuit boards. In a circuit forming technology using a liquid discharging method, technical development has also been advanced to form higher-definition patterns. The use of the liquid discharging method generally allows a circuit pattern to be recorded with a line width of approximately ten to tens of microns. In addition, there has been a study report that pattern formation on the order of submicrons is possible.
However, as the integration level thus increases, it becomes more difficult to maintain a high recording accuracy, for the following reasons:
(1) In order to form a high-definition circuit pattern, the lines of the pattern are required to be thinner, thus the number of droplets discharged from each discharge opening is reduced. Therefore, the size of each discharge opening is reduced. The size reduction makes the discharge opening more susceptible to clogging.
(2) When the solution is discharged from the discharge opening, minute droplets (hereinafter referred to as mist) are produced besides main droplets. The influence of the mist increases as the amount of solution to be discharged decreases. Parts of the mist consecutively stick to the adjacency of the discharge opening, and this causes dot misalignment.
(3) For example, even if a foreign substance does not stick, slight dot misalignment easily occurs between a plurality of discharge openings because of the structure of the recording head, that is, the manufacturing error of the discharging openings and foam or a substance sticking to a part of the inner wall of the discharging opening. With the reduction in the line width, the influence of such dot misalignment has become too serious to be ignored.
That is, when high-definition circuits are formed by the liquid discharging method according to the recent demand, not only the above-described problems (1) and (2) are further worsened, but also the new problem (3) is caused.
The method disclosed in the above-described publication can detect the presence or absence of an unrecorded portion in one line pattern, but cannot detect electrical connection (short circuit), that is, the contact between adjacent patterns due to position error of the discharging direction.